token

2. (Or "pumpkin") An abstact concept passed
between cooperating agents to ensure synchronised access to a
shared resource. Such a token is never duplicated or
destroyed (unless the resource is) and whoever has the token
has exclusive access to the resource it controls. See for
example token ring.

If several programmers are working on a program, one
programmer will "have the token" at any time, meaning that
only he can change the program whereas others can only read
it. If someone else wants to modify it he must first obtain
the token.